The overall process of hemofiltration which is carried out at the same time as preparation of a replacement fluid is shown, for example, in European Patent Application Nos. EP 0 042 939 and EP 0 087 171, respectively.
Plasmapheresis differs from hemofiltration primarily in that it requires a slightly more permeable membrane material so that even larger molecules can be filtered out therewith.
Examples of suitable hemofiltration membranes are described in Eurpoean Patent Application No. EP 0 046 816. Similarly, suitable membranes for plasmapheresis are described in European Patent Application No. EP 0 044 958. It will be clear, however, to those of ordinary skill in this art, that other membranes can also be used in conjunction with the realization of the present invention.
When in the past it has been necessary to measure the amounts of filtrate withdrawn in conjunction with processes such as hemofiltration or plasmapheresis it has been found to be quite difficult to carry out exact measurements due to variations in the pressure conditions and consequent varying flow conditions therein. In particular, specific problems were encountered on application of peristaltic pumps normally used in such processes. These types of pumps act from the outside of flexible blood tubes in a pulsating manner.
In accordance with a preferred embodiment of the apparatus of the present invention, a pressure equalizing device which comprises a gas-permeable but liquid-tight membrane is utilized. In this regard, reference is thus made to U.S. Pat. No. 4,198,971 which describes a drip chamber having a similar design. Such a drip chamber is also shown in European Patent No. 0 062 913. It again, however, does not relate to an overall system for measuring and replacing such fluids in connection with a filter or the like. Finally, German Patent Application No. DE 3243523A1 does show such a filtration system, which in this case includes a pair of expansion chambers 7 and 12 along with pumps 9 and 18 which are used to pump similar amounts of liquid to the liquid which is being filtered and discharged as well as replacement liquid.